Different transparent and light diffusing styrene copolymer compositions are known in the prior art. Thermoplastic polymer compositions comprising small polymer particles, formed in a variety of shapes and sizes (polymer beads) are also known.
US 2006/0100322 relates to a light diffusion resin composition, which is prepared by adding a bead type light diffusion resin comprised of PMMA as a light diffuser to a polyethylene terephthalate (PET) polyester resin. The light diffusion resin composition can be readily molded into a cover for lightings or sign boards by a processing method.
US-A 2009/0239985 discloses a rubber-modified styrene-based resin comprising a styrene-based polymer forming a matrix wherein rubber-like polymer particles are dispersed into like islands. However, the molding compositions of US 2009/0239985 show high yellow indices and low transmittance.
In KR 2008/0105477 and KR 2009/0054913 resins comprising a diffusion agent and a ter-polymer containing methyl methacrylate and styrene are disclosed. The diffusion agent may be a siloxane-based polymer, an acrylic polymer, or calcium carbonate. The optical transmittance of the resins is less than 62%.
Diffusion beads with a core-shell structure are described in US 2008/0299393. The core-shell structure consists of organic and inorganic material. A coated PET sheet is disclosed.
In JP-H06 220290 (Sekisui Plastics, 1994) a cross-linked methyl methacrylate polymer for the use as light-diffuser in a polymethyl methacrylate resin is described. The average particle diameter of the light-diffuser is between 1 and 30 micro meter (μm).
JP-A 2012/2146689 relates to light diffusing resin, comprising a basis resins such as polycarbonate, polymer particles such as cross-linked (meth)acrylic resins, an organic acid metal salt compound and a fluoropolymer.
EP-A 0634445 discloses a light diffuser composition containing inorganic particles and polymeric particles having a core-shell morphology. A light diffuser having a core of poly(butyl acrylate) and a shell of poly(methyl methacrylate) is tested. Polycarbonate compositions comprising these core-shell particles and optionally titanium dioxide are shown.
CN-A 103193915 relates to a light diffusing agent, a PMMA light diffusing plate and a manufacturing method thereof. The light diffusing plate comprises 0.5-3.0% by weight of light diffusing agent and 97.0-99.5% by weight of polymethyl methacrylate. The light diffusing agent is a random copolymer of styrene and methacryloxy-propylisobutyl polyhedral oligomer silsesquioxane.
In TW-A 2010/26766 a styrene-based resin composition is described, which contains a cross-linked resin particulate comprised of a (meth)acrylic acid ester system having a volume-average particle diameter (dv) of 0.7-2.5 μm and a ratio (dv/dn) of the volume-average particle diameter (dv) to the number-average particle diameter (dn) of 1.2 or less, wherein the charge crosslinking point equivalent weight is at least 0.15 meq/g and. The cross-linked resins comprise B(a) and B(b), wherein B(b) contains hydrolyzable silyl groups. The yellow indices of the resins according to the examples of TW 201026766 are greater than 8.2.
A styrene-based resin for light diffusion plates and a method for producing the same are disclosed in JP-A 2009/215476. The light diffusing agent is a blend of polymethyl methacrylate and a styrene-based resin. A methanol soluble content is used.
US 2009/0080079 discloses a light-diffusive aromatic polycarbonate-based resin composition. The composition contains a light-diffusing agent, which may be based on a silicone resin and styrene-(meth)acrylic ester copolymers.
U.S. Pat. No. 5,696,187 proposes a styrene methyl methacrylate (SMMA) copolymer resin composition, further comprising lithium sulfonate having a C8-16 alkyl group and inorganic glass beads as a light diffuser. The glass beads may optionally be coated with wax to avoid agglomeration of the glass beads in the resin.
Various applications, such as luminaires for LEDs or light diffusion sheets for flat screen or outdoor applications such as ship lightings, require materials with high light transmittance, high wide angle scattering (haze) and a low Yellowness Index (YI). So far, no satisfactory styrene-based compositions having these features are known. Ideally, the material should be stable and not sensitive towards hydrolysis. Especially for an application as luminaire materials for LEDs spatial scattering behavior is an important property, which may be quantified with diffusion factor.
One objective of the present invention is to provide a material with high light transmittance, high wide angle scattering (haze), low Yellowness Index (YI), and a tunable diffusion factor (OF). Furthermore, the use of bis-phenol A (BPA) should be avoided, which e.g. is often used in polycarbonate compositions.
It now was found that styrene-based composition can be used in extrusion or injection molding processes to produce the desired molding compositions comprising a styrene-based polymer and PMMA-based, cross-linked polymer beads. Surprisingly, it has further been found that the copolymer compositions of the present invention also bear a beneficial weather resistance.